In communication networks, advanced modulation formats are essential ingredients for carrying out efficient data transmissions. Due to the variety of transmission media in use and the variety of the disruptions that may affect the signals, there is no universally superior modulation format. In the field of optical transmissions, the search for higher data bitrates, for example of 100 Gbit/s/channel, has revealed that quadrature phase-shift keying (QPSK) modulation formats have very good performance.
To improve the flexibility of communication networks, one possible avenue is to design variable-bitrate data transmitters. Several solutions have been proposed to do this. For example, the document “Bit-Rate-Flexible All-Optical OFDM Transceiver Using Variable Multi-Carrier Source and DQPSK/DPSK Mixed Multiplexing”, K. Yonenaga et al., in Proceedings of OFC/NFOEC 2009 discloses an OFDM optical transmitter in which the data bitrate may be adjusted by modifying the number of subcarrier signals and/or by selecting the modulation format of each subcarrier from between a DQPSK (differential quadrature phase-shift keying) format and a DPSK (differential phase-shift keying) format.